Polypropylene is an excellent material for use in a variety of applications, particularly medical and food packaging applications. Polypropylene is shatter resistant, resistant to most chemical agents, inexpensive, easily formed, easily handled, and may be incinerated or recycled. Currently available polypropylene is subject to certain limitations, however. For example, polypropylene materials tend to be somewhat cloudy or translucent rather than clear. Also, typical polypropylene tends to soften and deform when sterilized at high temperature by steam or turn yellow and/or become brittle when treated with high energy radiation, particularly beta and gamma radiation.
Beta radiation, such as from an electron beam, or gamma radiation, such as from a cobalt-60 source, is often used to sterilize medical equipment. This is a particularly convenient means of sterilization since the items may be packed in bulk, or in individually sealed clean packages, and irradiated after packaging. Such treatments yield sterile instruments and devices without the need for special handling or repackaging after sterilization. Thus, sterility and enhanced patient safety are assured. However, because polypropylene tends to degrade when exposed to sterilizing levels of radiation, such treatment is generally inappropriate for medical devices incorporating polypropylene components.
But for this limitation, polypropylene would be very useful for making a tremendous number of useful items including syringe barrels, culture dishes, tissue culture bottles, intravenous catheters and tubing, and bags or bottles, surgical probes, suture material, and other goods.
The potential usefulness of polypropylene has been recognized for some time. Others in the field have attempted to overcome the property limitations by numerous means. In U.S. Pat. No. 4,110,185, for example, Williams, Dunn, and Stannett describe the use of a non-crystalline mobilizing agent in polypropylene formulations to increase the free volume of the polymer and prevent radiation embrittlement. In U.S. Pat. No. 4,845,137, Williams and Titus describe a polypropylene composition which is stable to sterilizing radiation, comprising polypropylene of narrow MWD, a liquid mobilizing additive, a hindered amine compound, and a clarifying agent. While these additives generally appear to enhance radiation-tolerance, mobilizing additives tend to be oily or greasy. This can contribute to processing difficulties and product flaws.
Other inventions attempting to stabilize polypropylene against the effects of high energy radiation employ syndiotactic polypropylene. EP-A2-0 431 475, describes making a radiation resistant polypropylene resin composition suitable for the preparation of molded articles in which physical properties "scarcely deteriorate during sterilization by radiation" by utilizing substantially syndiotactic polypropylene. The composition may also include a phosphorous containing anti-oxidant, an amine containing antioxidant, and a nucleating agent.
JP 04-214709 apparently describes ethylene/propylene copolymers with at least 50% syndiotacticity which have improved radiation tolerance. Such copolymers are produced by specific chiral metallocene-type catalysis and are preferably compounded with phosphorous or amine-containing antioxidants for best radiation tolerance.
U.S. Pat. No. 5,340,848 describes a radiation resistant polypropylene resin composition comprising a polypropylene having a substantially syndiotactic structure with optional anti-oxidants and/or nucleating agents.
WO 92/14784 describes blends of from 30 to 40 weight percent of an ethylene-based copolymer with 70 to 30 weight percent of a propylene-based copolymer for use in heat seal applications.
The prior art makes it clear that a simple, cost effective system to provide radiation tolerant polypropylene has long been sought. Ideally, such a polypropylene composition would provide products that are clear and would be dimensionally stable at elevated temperatures. Such products could optionally be subjected to sterilization by means other than radiation without softening or deformation or significant deterioration of optical properties. It would further benefit the makers of polypropylene articles if the polymer blend used for forming would not tend to foul the molding equipment with oil or grease. Users of the final formed products, as well as makers of such articles would benefit if such polymer compounds would not exude oil or grease from the surface of molded parts. Such articles would be particularly attractive to the medical and food packaging industries. Our invention provides such a simple and commercially practical system.